Extending energy optimization in goal-directed aiming from movement kinematics to joint angles


Goal-directed aiming movements are organized in a manner that optimizes speed, accuracy and energy expenditure. Energy optimization has been demonstrated as an undershoot bias in primary submovement endpoint locations, especially in conditions where corrections to target overshoots must be made against gravity. Two-component models of upper limb movement have not yet considered how joint angle displacements are organized to deal with the energy constraints associated with moving the upper limb in aiming tasks. This study was performed to address this limitation. Participants performed aiming movements to near, middle and far targets in the up and down directions with the index finger and two types of rod extensions (short and long) attached to the index finger. Movements with the rod extensions were expected to invoke different energy optimizing strategies in the up and down directions by allowing distal joints the opportunity to contribute to end effector displacement. Primary submovements undershot the far target to a greater extent in the downward direction compared to the upward direction, showing that movement kinematics show energy optimization in a manner that considers the effects of gravity. Importantly, as rod length increased, shoulder elevation was minimized in movements to the far up target and elbow extension was minimized in movements to the far down target. Contrary to our expectations, distal joints were not employed in either movement direction to optimize energy expenditure. While the overall results suggest energy optimization in the control of joint angles, they appear to be independent of the force of gravity.

Acknowledgments: NSERC